Electrophotographic image forming apparatus have been utilized not only for copying machines but also generally for printers as output means of computers, etc. for which demand has been increased remarkably in recent years. In the electrophotographic image forming apparatus, toner images are formed by uniformly charging a photosensitive layer of an electrophotographic photoreceptor provided to the apparatus by a charger, exposing the same, for example, by a laser light corresponding to image information, and supplying a particulate developer referred to as a toner from a developing device to electrostatic latent images formed by exposure.
While toner images formed by deposition of the toner as an ingredient of a developer to the surface of the electrophotographic photoreceptor is transferred by transfer means to a transfer material such as recording paper. However, not all the toner on the surface of the electrophotographic photoreceptor is transferred to the recording paper but the toner partially remains on the surface of the electrophotographic photoreceptor. Further, paper dusts of recording paper in contact with the electrophotographic photoreceptor during development may sometimes remain being deposited to the electrophotographic photoreceptor as they are.
Since the residual toner and the deposited paper dusts on the surface of the electrophotographic photoreceptor give undesired effects on the quality of images to be formed, they are removed by a cleaning device. Further, a cleanerless technique has been developed in recent years and they are removed by a so-called development and cleaning system of recovering the residual toner by a cleaning function added to the developing means without providing independent cleaning means. For the electrophotographic photoreceptor, since operations of charging, exposure, development, transfer, cleaning and charge elimination are conducted repetitively, a durability to electrical and mechanical external forces has been demanded. Specifically, it has been required for durability against abrasion or injury occurred upon frictional rubbing to the surface of the electrophotographic photoreceptor or against degradation of the surface layer caused by deposition of active substances such as ozone or NOx generated upon charging by the charger.
For attaining the reduction of cost and free of maintenance in the electrophotographic image forming apparatus, it is important that the electrophotographic photoreceptor has sufficient durability and can operate stably for a long time. The physical property of the surface layer constituting the electrophotographic photoreceptor has a great concern with the durability and the long time stability of operation of the electrophotographic photoreceptor.
Hardness is one of indices for generally evaluating physical properties of the materials, particularly, mechanical properties, not being restricted only to the physical property on the surface of an electrophotographic photoreceptor. The hardness is defined as a stress from a material against intrusion of an indenter. An attempt of quantitizing the mechanical property of a film that constitutes the surface of the electrophotographic photoreceptor by using the hardness as a physical parameter for recognizing the physical property of materials has been conducted. For example, scratch resistant test, pencil hardness test and Vickers hardness test, etc. have been generally known as the test method for measuring the hardness.
However, each of the hardness tests described above involves a problem in measuring the mechanical properties of a material showing complicate behaviors of plasticity, elasticity (also including retarded component) and creeping property in combination such a film comprising or organic material. For example, while Vickers hardness is used for the evaluation of hardness of a film by measuring the length of an indentation, this reflects only the plasticity of the film and can not exactly evaluate the mechanical property such as of those comprising an organic material showing a deformation state also including elastic deformation at a large ratio. Accordingly, the mechanical property of a film constituted with an organic material has to be evaluated in view of various properties.
One of prior arts for evaluating the physical property of the surface layer of the electrophotographic photoreceptor having the organic photosensitive layer proposes the use of a universal hardness value (Hu) and plastic deformation ratio according to the universal hardness test as specified in DIN 50359-1 (for example, refer to the publication of Japanese Unexamined Patent Publication JP-A 2000-10320). This prior art discloses that mechanical degradation less occurs to the surface layer of the photoreceptor when defining Hu and plastic deformation ratio to a specified range. However, substantially all light sensitive bodies having charge transporting layers using polymeric binders generally used at present are included in the definition range for the elasticity disclosed in JP-A 2000-10320 and this results in a problem that a suitable range is not defined substantially.
Further, in another prior art for evaluating the physical property of the surface layer of the electrophotographic photoreceptor, it has been disclosed that the scratch resistance of the photoreceptor can be improved by defining the Young's modulus as the mechanical property other than the hardness to a specified range together with the universal hardness value (Hu) described above in the photoreceptor provided to electrophotographic image forming apparatus using a contact charging process (for example refer to the publication of Japanese Unexamined Patent Publication JP-A 2001-125298).
However, another prior art is restricted to the case of using the contact charging process. In the electrophotographic system using an electrophotographic photoreceptor for image formation, the process for charging the photoreceptor is generally classified into two types, i.e., contact charging as disclosed in another prior art and non-contact charging using, for example, a scorotron. Accordingly, a difference is naturally present between the contact charging and non-contact charging due to the difference of the charging mode, for the performance required for the photoreceptor used respectively in them. This results in a problem that a suitable range for defining the surface physical property value for the electrophotographic photoreceptor using the contact type charging process can not be applied as it is to the surface physical property of the electrophotographic photoreceptor using the non-contact type charging process.